As of April 2008, Highway 4 officially does not exist within the City of London except south of Glanworth Drive, but instead is now signed with green "TO 4" trailblazer crown signs. The City was considering alternate routes for Highway 4 through the city in the early 2000s but this never materialized. Further north in Clinton, Highway 4 ends at the southern limits, though it remains signed.

The King's Highway 4 was originally designated in 1920 when the provincial government assumed the road running from Talbotville Royal to Elginfield through London. In 1927, the highway was further extended with the assumption of the road from Elginfield to Clinton. In the 1930s, the highway was again extended as far north as the intersection of Highway 10 in Flesherton, and south to Edith Cavell Boulevard in Port Stanley. In the 1970s, Highway 4 reached its maximum length when it was extended from Flesherton to Singhampton. However, in 1998, the Mike Harris government, seeking to balance the provincial budget, off-loaded responsibility for the Clinton–Singhampton and Talbotville-Port Stanley sections of Highway 4—almost 65% of its total length—back onto municipal governments.[3]